A typical electro-optical detector consists of a photoemitting diode with a photocathode and anode aligned parallel to one another, separated by a gap, and sealed inside a closed chamber. The photoemitting diode is aligned to face the source of electromagnetic radiation to be measured. The electromagnetic radiation impacts the photocathode, and imparts enough energy to it to dislodge electrons from the inside surface of the photocathode which faces the anode. From an external voltage source, a voltage difference is applied across the photocathode and anode, resulting in migration of the freed electrons across the gap from the photocarthode to the anode. Upon application of an external voltage potential across the electrodes, the photoelectrons move from the photocathode across the gap to the anode, thus giving rise to an electric current through photoconductivity. By suitable measuring techniques the photoelectric current is measured, thus providing desired information about the source of the electromagnetic radiation.
Photoconductive detectors and photoemissive detectors are two typical electro-optical detectors now in use. The photoconductive detector operates in the visible light spectrum; it does not operate in the infrared (IR) spectrum because IR radiation does not have sufficient photon energy to induce the photosensitive surface to emit electrons. In the photoconductive detector, the same material absorbs the light, uses its energy to produce electrons by internal ionization, and conducts a photo current by moving these photoelectrons by application of a voltage difference across the detector. The photoconductive material (that is, the material through which the photoelectrons travel from the photocathode to the anode, thus establishing the photoelectric current) must be almost devoid of electrons at the beginning of its operation, in order not to overload the light-induced signal with a large "dark" current produced by pre-existing electrons within the medium. "Dark current" as used herein is defined as that current which flows in the absence of light.
In the photoconductive detector, to obtain a photoconductive material which is almost devoid of electrons, semiconductor crystals have been widely used as both the detector material to detect the incident electromagnetic radiation and as the material for conducting the resultant electric current. However, in such a material the dark current is not zero, since the thermal agitation of the semiconductor crystal lattice has energies comparable to infrared photons and thus produces electrons by itself which flow as an electric current. The fluctuation in the dark current is a noise, and to make this noise negligible compared with the ambient infrared emission shot noise that limits an ideal detector of electromagnetic radiation, the detector must by cryogenically cooled. Use of cryogenic cooling systems raise considerable cost, bulk, and logistic problems. This is a disadvantage in that these photodetecting diodes are widely used in airborne and spaceborne systems, where weight and logistics are crucial considerations.
Another type of electro-optical detector, as mentioned above, is the photoemissive detector. In the photoemissive detector, the detection and current functions are separated. The photocathode detects the incident electromagnetic radiation and dislodges electrons from its interior surface into the material sandwiched between the photocathode and the anode; the material functions as the conductor of this photoelectric current. Such a division of labor permits the use of, among other things, photocathodes having higher light-absorption capabilities, and therefore requiring smaller thickness to absorb all of the incident light. This smaller thickness means a smaller photocathode volume for the same light-collecting area. Because of this smaller volume, the photocathode has a smaller amount of internal thermal agitation than the semi-conductor devices (the photoconductive detector device previously mentioned), and therefore has a smaller dark current at the same temperature than would the semi-conductor device, or provides equivalent performance with less cooling when sensitive to the same wavelength. Cryogenic cooling is rarely required with these devices, but they do not function in the infrared spectrum.
"Work function" as used here is defined as the amount of energy required to dislodge and transport a photoelectron from the photocathode surface into the gap between it and the anode. The lower the work function, the lower the lowest energy (or the higher the longest wavelength) that can still be observed by the photoemitting diode as a detector of electromagnetic radiation. It is thus desirable to lower the work function to cover the widest possible range of wavelengths. It has been extremely difficult, if not impossible, to find a photoemitting material whose work function into vacuum is much lower than 1.0 eV, which sets an upper limit of approximately 1.2 microns wavelength on the long wavelength threshold of a photoemissive detector.